Fullerenes are closed-cage carbon molecules composed of carbon-containing pentagons and hexagons. The discovery of Buckminsterfullerene, a C60 spherical allotrope of carbon, in 1985 by Kroto, et al. (“C60 Buckminsterfullerene”; Nature 318:162 (November 1985)) precipitated a flurry of activity directed towards understanding the nature and properties of fullerenes, particularly their use in synthetic chemistry and as electron acceptors, radical scavengers, non-linear optical limiters, and in many other applications. This research and development has been significantly hampered by the difficulty in obtaining large quantities of pure materials.
To date, fullerenes have been synthesized using a laser to ablate graphite, burning graphite in a furnace or by producing an arc across two graphite electrodes in an inert atmosphere. Other methods applied to synthesize fullerenes include negative ion/desorption chemical ionization and combustion of a fullerene-forming fuel. At present, combustion is the only method used for high volume production. In each method, condensable matter comprising a mixture of soot, other insoluble condensed matter, C60, C70, and higher as well as lower numbered fullerenes, and polycyclic aromatic hydrocarbons (PAH) in varying amounts is collected, with the total fullerene fraction typically between 5 and 15% of the total material collected, with the soot being 80%-95% of the remaining total material.
The procedures most commonly used for purifying fullerenes employ significant amounts of organic solvents. The solvents are used to first extract a fullerene mixture from insoluble soot and other insoluble condensed materials and then are used to purify and separate the individual fullerenes. Typically, the different constituents of the condensed matter are collected by filtration or some other technique, and the soluble components are extracted by a high energy-input extraction process such as sonication or soxhlet extraction using an organic solvent such as toluene. The extraction solution is then typically filtered to eliminate the particulate matter, and then purified by high performance liquid chromatography (HPLC), which separates the fullerenes from soluble impurities, such as polycyclic aromatic hydrocarbons (PAH) and aliphatic species, as well as separating individual fullerene species from other fullerene species.
The methods described above have a number of drawbacks. Organic solvents are expensive and must be disposed of as hazardous waste. HPLC also is expensive due to the high costs of equipment and stationary phase material, and the long time required. Furthermore, handling of the condensed matter for the separation stages can become difficult at larger scales due to the very small particle size of the soot particles (typically in the micron (μm) size range or less), and separation of liquid-borne soot particles is difficult and inefficient for particles in this size range.
Sublimation has also been conceptually demonstrated as a method to purify fullerenes from fullerene extract from arc processes (Dresselhaus et al, “Science of Fullerenes and Carbon Nanotubes,” Academic Press, San Diego, p. 118.), and is used to obtain high purity fullerenes from lower purity grades (e.g. 99.9% C60 from 99%) Sublimation methods that have been demonstrated operate on collected particulate or condensed matter or collected enriched fullerene product to purify fullerenes by addition of energy through heating (usually 500-1000° C.) at low pressures to dissociate the fullerenes from non-fullerene condensed matter. The vaporized fullerenes are then condensed onto a surface. Energy is required to dissociate fullerenes from a condensate when sublimation is used, material handling is costly, and irreversible losses of fullerenes occur (typically 20%) relative to the recovery of solvent extraction methods.
Fullerenes are typically found embedded in the collected soot particles of the condensed matter (Dresselhaus et al, “Science of Fullerenes and Carbon Nanotubes,” Academic Press, San Diego, p. 111). Transmission electron micrographs show that fullerene structures exist on the periphery of and within soot particles collected from a flame (Goel et al., “Combustion Synthesis of Fullerenes and Fullerenic Nanostructures” Carbon 40:177 (2002)). It is unclear in the art at which stage in the formation and collection process the embedding of fullerenes into soot particles occurs.
Laser ablation can liberate from soot and soot precursor particles trace amounts of fullerenes that were produced by processes not known to produce fullerenes (Reilly et al., “Fullerene Evolution in Flame-Generated Soot,” J. Am. Chem. Soc., 112:11596 (2000)). This observation is consistent with the formation of fullerenes in the condensed phase, i.e., in or on solid particles. Baum et al. in “Fullerene Ions and Their Relation to PAH and Soot in Low-Pressure Hydrocarbon Flames” (Ber. Bunsenges. Phys. Chem. 96:841 (1992)) postulate that fullerenes form in the condensed phase. The formation of fullerenes in the condensed phase could explain how fullerenes are found to be embedded in the solid particles.
There is also evidence that fullerenes are consumed by soot in a kinetically driven process, possible including chemical reaction, during the fullerene formation process (Grieco et al. in “Fullerenic Carbon in Combustion-Generated Soot,” Carbon 38:597 (2000)).
Homann describes spectroscopic in-situ flame observations of fullerenes in trace quantities in non-sooting or low-sooting flames (Gehardt et al., “Polyhedral Caron Ions in Hydrocarbon Flames,” Chem. Phys. Lett. 137:306 (1987)). Since little or no soot or other solid particulate matter is present in these flames, unlike the flame conditions typically used to produce fullerenes that produce significant amounts of soot, it is not clear from Gehardt et al. whether a significant fraction of fullerenes would be present as gaseous molecules during the formation process before they become embedded in the soot.
The literature on the combustion synthesis of fullerenes teaches that fullerenes are collected along with the soot with which they are associated in the flame, and that the fullerenes must be separated from the soot in post-collection process steps (Howard et al., Nature 352:139 (1991); Howard et al., J. Phys. Chem. 96:6657 (1992); McKinnon et al., Combustion and Flame 88:102 (1992); Richter et al., J. Chimie Physique 92:1272 (1995)).
In summary, it is not known whether fullerenes are formed in the condensed phase and so exist embedded in the solid particles, or whether they are formed in the gas phase and subsequently consumed by and/or embedded within the soot particles or agglomerates. Methods in the current art involve energy addition in solvent extraction, sublimation or other post-formation process steps to release the embedded fullerenes.
Lower cost and more effective methods for the separation and purification of fullerenes are desired.